Design of Diplexer with Direct Synthesis Method for Ground Penetrating Radar

This paper proposes a direct synthesis method for diplexer with wide fractional bandwidth (115.4%) of the second passband and a narrow isolation bandwidth (190 MHz) between the two passbands in the frequency domain and almost no tailings of the output signals in the time domain. This diplexer is designed for pulsed ground penetrating radar (GPR), which takes both the time domain and frequency domain responses into consideration. This method is able to synthesize diplexers with low insertion loss and high rejection in the frequency domain while avoiding tailing of the output signal in the time domain. Additionally, a T-junction with alternated stepped impedance line (SIL) is proposed, this significantly reduces the length of the junction by 40% and at the same time widens the bandwidth of the second passband by 78.6%. As a design example, a diplexer operating at DC-540 MHz and 730-2720 MHz is designed and fabricated, showing IL of 0.9 dB and rejection of 24 dB, while the width of the signal in the time domain is unchanged. The measurement results are in good agreement with the simulated ones, proving the effectiveness of the design.